1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device.
2. Description of the Related Art
In recent years, there has been extensive production of semiconductor devices in which a thin film transistor (hereinafter, also referred to as a TFT) is formed over a substrate which has an insulating surface such as a glass substrate and used as a switching element or the like. A structure of the thin film transistor has been proposed in which a semiconductor film having an island shape is formed over a substrate having an insulating surface by a CVD method, a photolithography method, or the like, and part of the semiconductor film having an island shape is used as a channel formation region of the transistor.
With such a backdrop, a method for manufacturing a thin film transistor in which a subthreshold swing (here, the subthreshold swing (S value) refers to the value of a gate voltage in a subthreshold region, which is required for changing drain current by one digit at a constant drain voltage) is decreased by thinning a thickness of a semiconductor film has been proposed in order to improve characteristics of a thin film transistor. However, by thinning the thickness of the semiconductor film, there is a problem in that resistance in the source region and the drain region and a contact resistance is increased which leads to reduction in on-current.
In order to solve these problems, a thin film transistor using a semiconductor layer in which a thickness of a channel formation region is made to be smaller than that of a source region or a drain region has been proposed (for example, Patent Document 1: Japanese Published Patent Application No. S61-48975 and Patent Document 2: Japanese Published Patent Application No. H5-110099, and Non-Patent Document 1: Electrochemical Society Proceedings Volume 98-22, PP. 204 to 220.